Last-Modified Header
The Last-Modified header is an HTTP response header used in web communications to indicate the date and time at which the origin server believes the resource was last modified. This information is crucial for several reasons:
- Caching: It helps in cache validation. Clients can use the Last-Modified date to check if their cached version of a resource is still up-to-date.
- Conditional Requests: Servers can respond with a status code of 304 (Not Modified) if a client sends a conditional request with an If-Modified-Since header that matches or is newer than the Last-Modified date, thus saving bandwidth.
Format
The value of the Last-Modified header is an HTTP-date timestamp, formatted according to RFC 7231, which typically looks like:
Last-Modified: Wed, 21 Oct 2020 07:28:00 GMT
History and Evolution
- The concept of Last-Modified dates goes back to the early days of the web, where HTTP/1.0 (1996) introduced mechanisms for conditional requests and cache validation.
- In HTTP/1.1, the specification was refined to include more precise definitions and handling of the Last-Modified header in RFC 2616, which was later updated by RFC 7232.
- Over time, as web technologies evolved, the use of Last-Modified has been complemented by other methods like ETag, which provides a more precise means of cache validation.
Contextual Use
The Last-Modified header:
- Is set by the server when a resource is requested, reflecting the time of the last change to the resource.
- Can be used by clients (like browsers or web caches) to determine if their local copy of a resource needs to be updated.
- Is not always accurate or present; for instance, dynamic content might not have a meaningful last modified time, or servers might not set this header for performance or privacy reasons.
Limitations and Considerations
- Granularity: The precision of Last-Modified is limited to the second, which can lead to issues if changes occur within the same second.
- Server-Side Configuration: Not all servers or content management systems correctly set this header, which can lead to inefficiencies in caching or outdated content being served.
- Comparison with ETag: While Last-Modified provides temporal information, ETag offers a more robust validation method since it's based on content rather than time.
Here are some external resources for further reading:
Related topics: